Admiral Ozzel was clumsy and stupid, as Darth Vader so nobly put it. It was Vader who killed Ozzel due to his incompetence just prior to the Battle of Hoth. Ozzel was an officer who went by the rules, but lacked creativity. Ozzel originally didn't think the Rebels were on Hoth at all. He later pulled the Imperial fleet out of hyperspace to close to the Hoth system, allowing the Rebels time to put up a deflector shield. This forced a ground invasion to take place, rather than a simple bombardment by the Imperial Star Destroyers. It was for these two reasons that Vader used the Force to kill Ozzel telekinetically allowing the more able Captain Piett to take over. Ozzel was truly a fool and he suffered dearly for it. Though had it not been for his foolish ways, we would never have seen the cool Hoth Battle sequence...